Constant frequency dc to dc converter with oscillation sustaining voltage regulation feedback loop

ABSTRACT

A self-oscillating DC to DC power converter establishes oscillations in the inverter through the voltage regulating feedback circuit. Its frequency of oscillation is regulated by a frequency regulation feedback circuit which controls the hysteresis response of the voltage regulating feedback circuit.

United States Patent Judd et a1.

[54] CONSTANT FREQUENCY DC TO DC CONVERTER WITH OSCILLATION SUSTAININGVOLTAGE REGULATION FEEDBACK LOOP [72] Inventors: Frank Fuller Judd,Madison; Jan Mark Lieberman, Lake Hiawatha; l-lelmut Wilhart, Whippany,all of NJ.

[73] Assignee: Bell Telephone Laboratories Incorporated,

Murray Hill, NJ.

[22] Filed: Dec.21,1970

[21] Appl.N0.: 100,151

[52] U.S.Cl ..32l/ 19,321/2 [51] Int. Cl. ..H02m 1/08 [58] Field ofSearch ..321/2, 18, 19

[56] References Cited UNITED STATES PATENTS 3,119,057 l/1964 Wilson..321/l9 Jan. 25, 1972 3,219,906 11/1965 Keller et al ..321/2 X3,325,716 6/1967 Gomi ..32l/2 3,373,334 3/1968 Geisz et a1. ....32 1/23,402,342 9/1968 Wagner .321/2 X 3,432,737 3/1969 Hunter et a1.....32l/2 3,461,374 8/1969 Rhyne, Jr ..32l/19 X Primary Examiner-WilliamM. Shoop, Jr.

- Attorney-R. J. Guenther and E. W. Adams, Jr.

57 ABSTRACT A self-oscillating DC to DC power converter establishesoscillations in the inverter through the voltage regulating feedbackcircuit. lts frequency of oscillation is regulated by a frequencyregulation feedback circuit which controls the hysteresis response ofthe voltage regulating feedback circuit.

5 Claims, 4 Drawing Figures PAIENIED 153251872 FIG. 1 w INVERTERFEEDBACK I 5 I I RECTIFIER FILTER 2 INVERTER 2I MODULATION CONTROL FIG.2

2 INVERTER REcTIFIER FILTER SWITCHING /25 VOLTAGE 22 MODOIRTION I EE8$CONTROL 2| '0 FIG. 3 I9 9 INVERTER g RECTIFIER FILTER SWITCHINGFREQUENCY J MODGERTION J25 To VOLTAGE CONTROL I CONVERTER 27 /29 F E UENHYSTERESIS Q J28 CONTROL DETECTOR VOLTAGE 22 ERROR DETECTOR J UDD LIEHERMAN ATTORNEY- CONSTANT FREQUENCY DC TO DC CONVERTER WITH OSCILLATIONSUSTAINING VOLTAGE REGULATION FEEDBACK LOOP BACKGROUND OF THE INVENTIONThis invention relates to DC to DC converters and, more particularly, toself-oscillating DC to DC converters. It is specifically concerned witha DC to DC converter in which oscillations are established by means offeedback signals transmitted via the voltage regulation feedback loop.

Conventional self-oscillating DC to DC converters, such as the onedisclosed in FIG. 1, utilize transformer feedback to generate thenonlinear feedback signals which establish the oscillations in theinverter circuit. The inverter feedback circuit 15 includes atransformer winding 16 magnetically coupled to the primary winding 1.]of the inverter 10. The inverter typically comprises push-pull coupledswitching devices which alternately conduct to invert a DC signal,applied to input terminals 1 and 2, to an AC signal at the outputtransformer winding 12. The inverter feedback generates nonlinearoscillating signals in response to the inverter output which sustain thealternate switching of these switching devices. L

The magnitude of the DC voltage output of the converter supplied to theoutput load 5 is controlled by a voltage regulation feedback loop 21including the modulation control which controls the duty cycle of theswitching devices in the inverter '10. The feedback arrangementestablishing oscilla-v tions in the inverter 10 is distinct andphysically separate from the voltage regulation feedback loop 21. Thisconventional converter circuit requires a multiwinding invertertransformer since additional windings are needed to supply the feedbacksignals establishing oscillations in the inverter.

It is therefore an object of the invention to establish the oscillationsin the inverter of a DC to DC converter by means of feedback signalssupplied by the voltage regulation loop.

The utilization of the voltage regulation feedback loop to establishoscillations in the inverter may cause the frequency of switching of theinverter to change in response to variations of several parameters, suchas the output load of the converter. A variation in frequency to a lowervalue than desired necessitates the utilization of a larger magnetizinginductance than would otherwise be needed in the outputtransformer ofthe inverter.

It is therefore another object of the invention to operate the invertercircuit of a self-oscillating converter at a constant frequencyindependently of variations in the output load or other parameters. v

It is yet another object of the invention to reduce the circuitcomplexity of a self-oscillating DC to DC converter by simplifying theinverter output transformer requirements and eliminating the need fordiscrete frequency control circuitry.

SUMMARY OF THE INVENTION f The above objects are achieved in accord withthe principles of the invention by utilizing the voltage regulationfeedback loop to establish self-oscilla,ions which, with the aid ofswitching logic circuitry, control the switching devices in the invertercircuit. The voltage regulation feedback loop responds to variations inthe output load voltage of the converter and feeds back control signalsto a bistable trigger circuit whose output controls the duty cycle ofthe active switching devices in the inverter. The output of the bistabletrigger in turn activates a binary signal steering circuit which iscoupled to and alternately transmits the switch drive signal to theoppositely phased switching devices in the inverter. This arrangementestablishes the nonlinear oscillations required to operate the inverter,in addition to controlling the duty cycle of the inverting switchingdevices.

The bistable trigger circuit, included in the voltage regulationfeedback loop, has a hysteresis response to the voltage feedback'signal.That is, the threshold of the input signal at which the bistable triggercircuit changes state is dependent upon the direction of change as wellas the magnitude of the input signal. The magnitude difference betweenthe threshold of a decreasing and the threshold of an increasing inputsignal is the hysteresis width of the bistable trigger circuit. Afrequency detector circuit monitors the switching frequency of theinverter and generates a frequency error signal which is utilized tocontrol the hysteresis width of the bistable trigger circuit. Thecontrol of the hysteresis width of the trigger circuit determines thefrequency at which the switching devices in the inverter circuitalternate.

It is apparent that this arrangement advantageously eliminates the needfor independent circuitry to establish oscillations in the invertercircuit and thus improves the response and economy of the DC to DCconverter of the invention over comparable converters in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be readilyunderstood with reference to the following detailed description and theattached drawings wherein FIG. 1 is a block diagram of a conventional DCto DC converter circuit and which is described hereinabove;

FIG. 2 is a block diagram of a DC to DC converter circuit in which theinverter oscillations are sustained by utilization of feedback signalsin the voltage regulation feedback loop.

FIG. 3 is a block diagram of a DC to DC converter in which the frequencyof switching in the inverter circuit is controlled by a frequencycontrol arrangement; and

FIG. 4 is a schematic diagram of a DC to DC converter circuit includingfeedback arrangements in accord with the principles of the invention toregulate the output voltage and frequency of switching and establishinverter oscillations in the DC to DC converter circuit.

DETAILED DESCRIPTION The voltage regulated DC to DC converter disclosedin FIG. 2 establishes the alternate switching of the inverter 10 bymeans of the feedback signals of the voltage regulation feedback loop.The DC voltage to be converted is coupled to the input terminals 1 and 2of the inverter circuit 10. The inverter 10, by means of switchingaction stimulated by the voltage regulation feedback loop, transformsthis DC voltage into an AC signal which appears at the transformersecondary winding 12. The transformer action permits the magnitude ofthe AC voltage to be changed to another magnitude. The AC voltage of thetransformer secondary 12 is rectified by a of 13. A low-pass filter 15blocks the transmission of high-frequency components of the rectifiedsignal to the output load 17.

The output voltage is monitored and controlled by means of a voltageregulation feedback loop 21 which is coupled to the output lead 14. Thevoltage regulation feedback loop 21 includes a voltage error detector 22and a switching and modulation control 25 which is coupled to theswitching devices of the inverter 10. The voltage error detector 22monitors the voltage at the output 14 and compares it to an internalreference voltage. An error voltage is generated from this comparisonand applied to the switching and modulation control 25.

The switching and' modulation control 25 comprises a bistable triggercircuit which functions as a level detector and logic circuitry to steerthe detector output signal to alternate ones of the switching devices ofthe inverter circuit 10. The bistable trigger circuit responds to theincrease and decrease of the voltage feedback signal by generating adrive signal with appropriate duty cycle for the active switchingdevices of the inverter 10. The logic circuitry applies this drivesignal to alternate ones of the switching devices of the inverter 10.

Variations in the output load and other parameters may have a directeffect on the switching frequency of the inverter 10. This isundesirable in a transformer-coupled converter since the invertertransformer is required to have sufficient magnetizing inductance toaccommodate the lowest switching frequency which is likely to beencountered. To permit efficient utilization of the'inverter transformerit is essential that the inverter be operated at a constant frequency.

The inverter of the DC to DC converter, disclosed in FIG. 3, is operatedat a substantially constant frequency by adding a frequency controlfeedbackcircuit to the feedback circuitry controlling the switching ofthe inverter circuit 10. The frequency control feedback circuit includesa frequency-tovoltage converter 27 coupled to the output 19 of therectifier 13 and a frequency error detector 28. The frequency errordetector 28 compares the voltage from converter 27 to aninternalreference voltage and generates a frequency error signal Itherefrom. This frequency error signal is applied to the hysteresiscontrol circuit 29 included in the switching ancl modulation control 25.The hysteresis control circuit 29 con- 7 trols the upper and lowertriggering signal levels at which the switching and modulation control25 responds to the error voltage signal of the voltage error detector22. Since thedifference between the upper and lower triggering levels orhysteresis width of the bistable trigger circuit included in theswitching and modulation control 25 is directly related to theoscillating frequency of the voltage regulator feedback circuit, it isapparent that through control of this hysteresis width the oscillatingfrequency of the voltage control feedback circuit 21 can be controlled.This frequency controladvantageously permits the use of the voltageregulation feedback loop 21 to sustain switching at a constant frequencyin the inverter circuit 10. 1

A self-oscillating DC to DC converter is shown schematically in F1G. 4in which a DC voltage; of one voltage level, applied to the inputterminals 1 and 2 isconverted to another voltage level at the outputterminals 3 and 4. The voltage levels are converted by transformeraction and duty cycle control of the inverter switching devices. Thealternate switching of the inverter switching transistors 70 and 80inverts the DC input voltage into an AC voltage by alternatelyenergizing the inverter transformer 17in opposite directions. Theswitching of transistors 70 and 80'is controlled by the switching andmodulation control 25 which applies bias signals to transistors 70 and80, via the driving transistors 50 and-60.

The frequency of the switching of transistors 70 and 80 is controlled inresponse to the frequency-'to-voltage converter 27" and the frequencyerror detector 28. The error signal generated by the frequency error.detector 28 is appliedto the hysteresis control 29 which is a part ofthe bistable trigger circuit contained in the switching and modulationcontrol 25 and serves to control its hysteretic response to the voltage.control feedback signals of the converter. The hysteresis width isautomatically altered continuously to maintain the inverter switchingfrequency of the converter at some constant value.

The principles of the invention may be readily understood by explainingthe operation of the voltage and frequency regulation feedback circuitsin the illustrative embodiment of the converter. The output voltage atterminals 3 and 4 of the converter is regulated in response to a voltageerror detector 22 which rnonitors the output voltage and compares it toan internally generated reference voltage. The voltage error detector 22generates an error voltage in response to this comparison which isproportional to the difference between the output and referencevoltages. The voltage error detector 22 comprises a potentiometer 18,which is shunted across the output terminals 3 and 4. A transistor 90has its base electrode 91 The conductivity of transistor 90 isproportional to the difference between the voltage at the wiper arm ofpotentiometer 18 and the reference voltage of breakdown diode 46.

The collector voltage of transistor 90 is applied, via the feedback lead21, to a hysteresis control circuit 29. The

hysteresis control circuit controls the magnitude of the feed- I backsignal, applied to leads 61 and 62 in the switching and modulationcontrol 25. These respective magnitudes control the hysteretic responseof the switching and modulation control 25 as described below.

The hysteresis control circuit 29 includes a bridge circuit comprisingthe resistors 73 and 75 and the Potentiometers 74 and 76. The impedanceof the potentiometers 74 and 76 is somewhat greater than the impedanceof the resistors 73 and 75. It is apparent that when the transistor 40whose collectoremitter path connects nodes 5 and 6 of the bridge is notconducting, the potential at node 5 will exceed the potential at node 6.With transistor 40 not conducting, the potentiometers 74 and 76 areadjusted so that the fractions of the feedback voltage applied to leads61 and 62 in the switching and modulation control 25 are just slightlydifferent. This is to establish the minimum possible hysteresis width atthe desired mean triggering level of the switching and modulationcontrol 25. It is apparent that conduction of transistor 40 will alwayscause the fraction of the feedback voltage applied to lead 61 to exceedthe fraction of the feedback voltage applied to lead 62.

The conductivity level of the transistor 40 connecting nodes 5 and 6 ofthe bridge controls the difference between the fractions of the feedbackvoltage applied to leads 61 and 62. The bistable switching circuit 66included in the switching and modulation control 25 in conjunction withthe hysteresis control 29 has a hysteretic switching characteristic aslong as the voltage at lead 61 always exceeds the voltage at lead 62. Itis apparent that by controlling the conductivity of transistor40, thehysteretic response of the bistable trigger circuit comprising thehysteresis control 29 and the bistable switching circuit 66 can bedirectly controlled. The diode 44 is included in the collector path oftransistor 40 to prevent the base drive current of transistor 40 fromreturning to ground via the collector path.

In addition to the hysteresis control 29 and the bistable switchingcircuit 66, the switching and modulation control 25 comprises a JKflip-flop 67, and two NOR-gates68 and 69 which function as a steeringcircuit to direct the output of the bistable switching circuit 66 to theappropriate driving transistors 50 and 60. TheNOR-gates 68 and 69 areoperated in response to the state of the output of the bistableswitching circuit 66 and the output of the JK flip-flop 67. The bistableswitching circuit 66 comprises the three NOR-gates 63, 64 and 65. Two ofthe NOR-gates 63 and 64 are cross-connected to form a set/resetflip-flop. A third NOR-gate 65 serves as an inverting input in one ofthe trigger lines of NOR-gate 63. As described above, the switchingcharacteristics of the bistable trigger circuit comprising the bistableswitching circuit 66 and the hysteresis control 29 has a hystereticresponse to the voltage regulation feedback signal as long as thefraction of the feedback voltage applied to lead 61 exceeds the fractionof the feedback voltage applied to lead 62. This hysteretic response maybe readily ascertained by a logical analysis of the sequential responseof the NOR-gates 63, 64 and 65 to the fractions of the feedback voltageapplied to leads 61 and 62. An understanding of the operation of thebistable switching circuit 66 may be readily acquired by an examinationof the following table which describes the response of the bistableswitching circuit 66 in terms of the equivalent logic values of theinput and output signals. A signal with a l logic value exceeds theinput threshold of the NOR gate while a 0 signal is below thisthreshold. The lower case letters at the top of the columns in thefollowing table identify the various leads in the switching circuitidentified by the same lower case letter in FIG. 4.

Terminals States a b c d c l 0 0 l l 0 2 l 0 0 1 0 3 1 1 0 0 1 4 l 0 0 0l 5 0 0 l l 0 The output signal at the terminal e or lead 87 of thebistable switching circuit 66 is applied to a JK flip-flop 67. The JKflipflop 67, whose output changes state in response to an input to thetoggle input lead 87, has its two output leads 88 and 89 connected tothe NOR-gates 68 and 69. The JK flip-flop 67 is a general-purposebistable trigger circuit which, under certain operating conditions, maybe used as a toggle switch, as used herein, by grounding its set andclear input terminals. The JK flip-flop is well known in the art and itis not believed necessary to describe it in detail. A description of theJ K flip-flop may be found in The Integrated Circuit Data Book, MotorolaSemiconductor Products Inc., 1st Edition, Aug. 1968, Pages -27.

The complementary outputs of the 1K flip-flop on leads 88 and 89 changestate every time the output of the bistable switching circuit 66 on lead87 assumes a low-voltage state, provided that the set and clear inputsto the flip-flop are grounded as shown in FIG. 4. Since the outputs ofthe J K flipflop on leads 88 and 89 are complementary, it is apparentthat the NOR-gates 68 and 69 are alternately enabled by signalsrepresentative of logic zeros to transmit signals to the base electrodes51 and 52 of transistors 50 and 60, respectively. The output terminalsof the enabled NOR-gates 68 and 69 alternately assume logic 1 levels inresponse to logic 0s appearing at lead 53 of the bistable switchingcircuit 66, thereby furnishing drive signals to the base electrodes 51and 52. This serves to alternately drive the transistors 50 and 60 intosaturation. The saturated transistors 50 and 60 in turn alternatelydrive the inverter switching transistors 70 and 80 into saturation. Withthe state of the JK flip-flop, as indicated by the respective l and 0shown within the enclosed box 67 in FIG. 4, the NORgate 68 is enabled torespond to a logic 0 on lead 53. As the output voltage across terminals3 and 4 decreases in value, the collector voltage of transistor 90increased due to its decreased conductivity. The fraction of thisincreasing voltage applied to the terminal 62 of the bistable switchingcircuit 66 crosses a threshold at which the outputs of the bistableswitching circuit change state. The output at terminal 87 will assume ahigh voltage state. The complementary output at terminal 53 will assumea low voltage state. This low voltage state causes the output ofNOR-gate 68 to assume a high voltage state thereby furnishing a drivesignal to the base 51 of the transistor 50. The transistor-50 is driveninto saturation and in turn drives the switching transistor 70 intosaturation.

With transistor 70 conducting the input voltage is applied to theprimary winding 11 of the transformer 17. The induced voltage appearingacross the secondary winding 12 is rectified by one of the diodes 96 and98 and applied across the output terminals 3 and 4. As the outputvoltage increases in magnitude, the collector voltage of transistor 90decreases. The fraction of this decreasing voltage applied to lead 61,which is determined by the hysteresis control, passes the thresholdlevel of gate 65 and causes the bistable switching circuit 66 to changestate. The bistable switching circuit 66 applies a low level signal tothe toggle input of the JK flip-flop 67 via lead 87 causing it to changestate, reversing the indicated 1 and 0 in FIG. 4. Hence the nexttransition to a low-voltage state occurring at lead 53 will cause theoutput of the now enabled NOR- gate 69 to assume a high-voltage statethereby furnishing a drive signal to the base 52 of transistor 60. Thetransistor 60 saturates and drives the alternate switching transistor 80into saturation. It is apparent from the preceding description that theindependent switching frequency of each of the transistors 70 and 80will be exactly one-half of the ripple frequency of i the output sighalof the converter at terminals 3 and 4.

As indicated above, the frequency of switching of the inverter of theconverter tends to vary with changes in the output load and with changesin the input voltage. These changes in frequency are undesirable becauseof the extra magnetizing inductance which is needed in the invertertransformer to accommodate the lowest frequency which is likely to beencountered. The frequency of switching hence is regulated by includingin the converter a frequency regulation feedback circuit which has thehysteresis control 29 in common with the voltage regulation feedbackloop, as described hereinbelow. The frequency regulation feedbackcircuitry is activated in response to the switching frequency which isdetected at the node 99. The node 99 is contiguous to the rectifierdiode 98 connected to the output transformer winding 12.

The pulse signal occurring at node 99 in response to the switching ofthe inverter is differentiated by the capacitor 97. The trigger signaloccurring therefrom is applied to the frequency-to-voltage converter 27.The frequency-to-voltage converter 27 comprises a monostable pulsingcircuit whose output is connected to an integrating circuit comprisingthe integrating resistor 45 and the integrating capacitor 46. Theaverage voltage across the capacitor 46 is directly proportional to theswitching frequency of the inverter. This voltage is applied to thefrequency error detector 28.

The frequency error detector 28 comprises the transistor 30 and thebreakdown diode 32. The breakdown diode 32, which is energized by theinput voltage, generates a reference voltage, an adjustable fraction ofwhich is coupled via potentiometer 33, to the base electrode 34 oftransistor 30. Potentiometer 33 thereby affords a means of adjusting theinverter switching frequency, as described hereinbelow. The voltageacross the capacitor 46 is coupled to the emitter 31 of the transistor30. The collector voltage of the transistor 30 is directly proportionalto the difference between the actual operating frequency of the inverterand the desired regulated frequency of the inverter which is establishedby the fraction of the magnitude of the breakdown voltage across thebreakdown diode 32, which is applied to the base electrode 34 oftransistor 30 via potentiometer 33.

The output voltage of the frequency error detector 28 is applied to thehysteresis control circuit 29. The magnitude of this voltage determinesthe hysteresis width established by the hysteresis control circuit 29.As described above, the system hysteresis of the converter is controlledto hold the switching frequency of the inverter constant. The frequencyerror signal of the frequency error detector 28 is applied to the base41 of transistor 40. The magnitude of this signal controls theconductivity of transistor 40 connecting nodes 5 and 6 of the bridgecircuit of the hysteresis control circuit 29. By controlling theconductivity of transistor 40, the hysteretic width between the upperand lower triggering levels of the bistable trigger circuit comprisingthe hysteresis control circuit 29 and the bistable switching circuit 66is controlled in response to the deviation of the switching frequency ofthe inverter from its regulated value.

It is apparent from the foregoing description that the voltage errorsignal fed back to the hysteresis control circuit 29 establishesoscillations in the inverter circuit and controls the duty cycle of theswitching transistors therein. The frequency regulation feedbackcircuit, through adjustment of the upper and lower triggering levels atwhich the switching and modulation control 25 responds, regulates thefrequency of switching at some desired value.

What is claimed is:

l. A self-oscillating DC to DC power converter comprising an invertercircuit including two switching devices,

a voltage regulating feedback circuit comprising a bistable switchingcircuit with two output terminals having complementary output states,

a J K flip-flop arranged to switch output states in response to a changein state of signal applied to a single input, said toggle inputconnected to one of said output-terminals of said bistable switchingcircuit,

a steering circuit comprising two signal gates connected in parallel tothe other output terminal of said bistable switching circuit, saidsignal gates being alternately enabled by the complementary outputs ofsaid JK flip-flop and the two gate outputs being connected to said twoswitching devices, respectively,

a frequency regulating feedback circuit coupled to the output of saidconverter and including a frequency-to-voltage converter, a referencevoltage to establish the regulated frequency, means to compare thevoltage output of said frequency-to-voltage converter and said referencevoltage to derive a frequency error signal therefrom, and

a hysteresis control circuit to control the upper and lower triggeringlevels of said bistable switching circuit and responsive to said meansto compare and derive whereby said voltage regulation feedback circuitestablishes the switching of said switching devices and said frequencyregulation feedback circuit regulates the switching frequency bycontrolling the hysteresis of the voltage regulation feedback circuit.

2. A self-oscillating DC to DC power converter as defined in claim 1wherein said hysteresis control circuit comprises a bridge circuit withpotentiometers in opposing branch arms, the wipers of saidpotentiometers coupled to two input leads of a bistable switchingcircuit, and

a variable impedance coupled across the bridge to control the voltagedifference between said two input leads of said bistable switchingcircuit. 3. A self-oscillating DC to DC power converter as defined inclaim 2 wherein said means to compare and derive comprises inverter,

4. A DC to DC converter circuit comprising input terminals,

output terminals, I

voltage monitoring means coupled to said output terminals,

an inverter circuit coupled to said input terminals including at leasttwo oppositely phased switching devices,

a feedback loop coupling said voltage monitoring means to said switchingdevices, said feedback loop including a hysteretic bistable triggermeans and at least two gating means to couple the output of said triggermeans to said switching devices,

a steering control toggle circuit to alternately enable said gatecircuits in order to alternately apply drive signals to theappropriately phased switching devices, and

a frequency detector coupled to said inverter circuit and meansresponsive to said frequency detector to alter the hysteretic responseof said bistable trigger means.

5. A DC to DC converter as defined in claim 4 wherein said bistabletrigger circuit includes a bistable switching circuit with two inputsand an input stage to couple different amplitudes of voltage feedbacksignals to said two inputs, and

said input stage responsive to said frequency detector to control therespective amplitudes of voltage applied to said two inputs.

1. A self-oscillating DC to DC power converter comprising an invertercircuit including two switching devices, a voltage regulating feedbackcircuit comprising a bistable switching circuit with two outputterminals having complementary output states, a JK flip-flop arranged toswitch output states in response to a change in state of signal appliedto a single input, said toggle input connected to one of said outputterminals of said bistable switching circuit, a steering circuitcomprising two signal gates connected in parallel to the other outputterminal of said bistable switching circuit, said signal gates beingalternately enabled by the complementary outputs of said JK flip-flopand the two gate outputs being connected to said two switching devices,respectively, a frequency regulating feedback circuit coupled to theoutput of said converter and including a frequency-to-voltage converter,a reference voltage to establish the regulated frequency, means tocompare the voltage output of said frequency-to-voltage converter andsaid reference voltage to derive a frequency error signal therefrom, anda hysteresis control circuit to control the upper and lower triggeringlevels of said bistable switching circuit and responsive to said meansto compare and derive whereby said voltage regulation feedback circuitestablishes the switching of said switching devices and said frequencyregulation feedback circuit regulates the switching frequency bycontrolling the hysteresis of the voltage regulation feedback circuit.2. A self-oscillating DC to DC power converter as defined in claim 1wherein said hysteresis control circuit comprises a bridge circuit withpotentiometers in opposing branch arms, the wipers of saidpotentiometers coupled to two input leads of a bistable switchingcircuit, and a variable impedance coupled across the bridge to controlthe voltage difference between said two input leads of said bistableswitching circuit.
 3. A self-oscillating DC to DC power converter asdefined in claim 2 wherein said means to compare and derive comprises abreakdown diode energized by the input to said inverter, and atransistor whose base is adjustably coupled to said breakdown diode andwhose emitter is coupled to said frequency-to-voltage converter.
 4. A DCto DC converter circuit comprising input terminals, output terminals,voltage monitoring mEans coupled to said output terminals, an invertercircuit coupled to said input terminals including at least twooppositely phased switching devices, a feedback loop coupling saidvoltage monitoring means to said switching devices, said feedback loopincluding a hysteretic bistable trigger means and at least two gatingmeans to couple the output of said trigger means to said switchingdevices, a steering control toggle circuit to alternately enable saidgate circuits in order to alternately apply drive signals to theappropriately phased switching devices, and a frequency detector coupledto said inverter circuit and means responsive to said frequency detectorto alter the hysteretic response of said bistable trigger means.
 5. A DCto DC converter as defined in claim 4 wherein said bistable triggercircuit includes a bistable switching circuit with two inputs and aninput stage to couple different amplitudes of voltage feedback signalsto said two inputs, and said input stage responsive to said frequencydetector to control the respective amplitudes of voltage applied to saidtwo inputs.